1. Field of the Invention
The invention relates to a hearing instrument comprising a circuit for transforming an audio signal into an output signal.
The invention further relates to a circuit for transforming an audio signal into an output signal and a method for transforming an audio signal into an output signal.
2. Description of Related Art
A hearing instrument according to the preamble is known from WO 96/12383. Hearing instruments are used to improve the perception of sounds by hearing impaired persons. In general, such hearing instruments try to transform a received audio signal into a stimulation signal in such a way that by stimulating a persons hearing system with that stimulation signal the hearing of a normal hearing person is approximated as closely as possible.
In the known hearing instrument the transformation of the received audio signal into the stimulation signal comprises the following steps: first, the audio signal is analysed in order to identify the presence of predefined portions, e.g. phonemes, tones or chords, therein. Next, on the basis of the identified portions the stimulation signal is determined, e.g. by using look up tables or by mere calculation, in such a way that the time domain response of a normal hearing person to those portions is approximated.
The transformation of the received audio signal in the known hearing instrument is relatively complex.
An object of the invention is to provide a hearing instrument, wherein the transformation of the received audio signal into the stimulation signal is performed in a relatively simple manner. This object is achieved in the hearing instrument according to the invention, which is characterized in that the circuit comprises emphasis means for emphasising in the output signal substantial intensity changes of the audio signal. The invention is based upon the recognition that the neural activity in a persons hearing system at the onset of a sound stimulus is much higher than the neural activity after the onset. The more suddenly the intensity of the stimulus increases, the higher this activity will be. The duration of this so called short term adaptation is in the order of magnitude of 10 ms. It is assumed that the origin of the adaptation effect is at the inner hair-cell/auditory-nerve synapse. For persons whose hearing system is damaged in such a way that this adaptation effect is not fully present the hearing instrument according to the invention provides an improved perception of sounds. Experiments have shown that, as this hearing instrument emphasises the onset of the typical intensity increases of speech, the speech intelligibility for users can be improved considerably.
An embodiment of the hearing instrument according to the invention is characterized in that the circuit further comprises filter means for filtering the audio signal, whereby the filter means are coupled to the emphasis means so that substantial intensity changes of the filtered audio signal are emphasised in the output signal. In order to approximate a persons hearing system, it can be advantageous to split the frequency spectrum of the received audio signal into several frequency bands or channels. For instance, if the hearing instrument comprises a cochlear implant this can be used to simulate the tonotopic organisation of the cochlea.
A further embodiment of the hearing instrument according to the invention is characterized in that the emphasis means comprise a peak generator for generating from the audio signal a peak signal and including means for including the peak signal in the output signal. By this measure sudden changes in the intensity of the audio signal are transformed into peak signals which are included in the output signal. The duration of these peak signals is preferably in the order of magnitude of 10 ms. The inclusion of the peak signals in the output signal emphasises the onset of substantial intensity changes in the audio signal.